prevent vtctld from creating tons of S3 connections

[rvrangel]

Description

This fixes the issue described in #15335. It seems the bug was introduced in #4200.

The default transport was changed in favour of one were we could skip validating S3 certificates, but instead of cloning the default transport and adjusting the settings, we are creating a new transport from scratch but not setting any defaults (see current Go defaults for DefaultTransport). By doing so, we don't set keepalives, timeouts, max idle connections, etc and other values in Transport and since some default to zero it basically means "no limit". To make matters worse, since we are creating a new transport for every client (in this case, for each request from what I understand), there is no sharing of connections between different requests, so the first call to GetBackups() will open some S3 connections and keep them on its private transport instead of sharing it across different requests.

After this fix, the number of established connections seems to be consistently below 5, and from initial testing it seems S3 connection errors seem to be gone.

Related Issue(s)

Checklist

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Deployment Notes

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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 65.44%. Comparing base (696fe0e) to head (13a3911). Report is 68 commits behind head on main.

Files	Patch %	Lines
go/vt/mysqlctl/s3backupstorage/s3.go	85.71%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15296      +/-   ##
==========================================
- Coverage   67.41%   65.44%   -1.98%     
==========================================
  Files        1560     1562       +2     
  Lines      192752   193914    +1162     
==========================================
- Hits       129952   126906    -3046     
- Misses      62800    67008    +4208     

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[rvrangel]

thanks @deepthi , I created #15335 and mentioned it here in the PR description

[deepthi]

This is a really great contribution @rvrangel! Thank you.